1. Field of the Invention
The present invention relates to a clamping circuit for a liquid crystal display device, and more particularly to a clamping circuit for a large-screen and high-definition color liquid crystal display device of an active-matrix type.
2. Description of the Prior Art
Since a liquid crystal display device (LCD) of an active-matrix type using thin film transistors has advantages that it can drive two-dimensionally arranged pixel electrodes independently of one another, can implement a large-screen, high-definition and fine-gradation image display device, can be made compact thanks to its flat panel, can be driven by a low voltage and can be low in power consumption, the demand on it has been more and more expanded.
Since a liquid crystal display device of this kind sets a picture signal at a voltage suitable for driving liquid crystal, a black level in a picture signal is clamped to a predetermined clamping voltage. Furthermore, since the characteristic of transmissivity of liquid crystal to voltage applied to the liquid crystal is steep in the vicinity of the black level, the degree of gradation is lowered in the vicinity of the black level. To this end, so called gamma correction is performed, in which a picture signal is amplified by changing the degree of amplification in response to a picture signal level, which is called a gamma correction, is performed.
A clamping circuit for a liquid crystal display device clamps in general the black levels of analog RGB (Red, Green and Blue) picture signals to the same voltage.
In recent years, however, incorrect operation of a gamma-correction circuit and a coloring problem caused by variation in black level after a gamma correction are getting a great deal of attention as a problem to be solved.
On the other hand, in order to prevent the coloring problem caused by the difference in the characteristic of applied voltage to brightness among R, G and B signals of a LCD panel, a clamping circuit for a conventional liquid crystal display device described in Japanese Patent Laid-Open Publication No.Sho 64-78,592 has proposed to set pedestal clamping levels so that at least one of them is different from the other according to R, G and B color signals.
Referring to FIG. 1, a conventional clamping circuit for a liquid crystal display device is provided with three clamping circuits connected between three coupling capacitors and three gamma-correction circuits, respectively. The coupling capacitors 1, 2 and 3 are provided for receiving input color signals R, G and B of a picture signal VI to remove a direct current (DC) bias. Output signals from the coupling capacitors 1, 2 and 3 are inputted into the clamp units 400, 500 and 600, respectively, to output clamped color signals RC, GC and BC to the gamma-correction circuits 7, 8 and 9, respectively. The clamped color signals RC, GC and BC are obtained by newly adding a predetermined clamping voltage, which is the pedestal level of the picture signal, to the AC component of each of the color signals R, G and B whose DC bias has been cut. The gamma-correction circuits 7, 8 and 9 perform a predetermined gamma correction and amplification on each of the clamped color signals RC, GC and BC to output the respective output color signals RG, GG and BG.
The clamp unit 400 is provided with a clamping portion 410 for performing a clamping operation of adding a clamping voltage to the AC component of an inputted color signal R and outputting the clamped color signal RC, and a clamping voltage generating circuit 420 for generating and supplying a predetermined clamping voltage to the clamp unit 410.
The clamp unit 500 is provided with a clamping portion 510 for performing a clamping operation of adding a clamping voltage to the AC component of the inputted color signal G and outputting the clamped color signal GC.
The clamp unit 600 is provided with a clamping portion 610 for performing a clamping operation of adding a clamping voltage to the AC component of an inputted color signal B and outputting the clamped color signal BC, and a clamping voltage generating circuit 620 for generating and supplying a predetermined clamping voltage to the clamp units 510 and 610.
The clamping voltage generating circuits 420 and 620 are of the same composition, and referring to FIG. 2 showing with a block diagram the composition of the clamping voltage generating circuit 420 for the color signal R as a representative, the clamping voltage generating circuit 420 is provided with a variable resistor 423 for generating a divided voltage FR by dividing variably the voltage VDD of a power source and a buffer circuit 422 consisting of a voltage follower circuit for buffer-amplifying the divided voltage FR and outputting a clamping voltage CR.
In the same way, the clamping voltage generating circuit 620 is provided with a variable resistor and a buffer circuit.
Now, operation of a clamping circuit of a conventional liquid crystal display circuit is described with reference to FIG. 1 and FIG. 2. The color signals R, G and B of the input picture signal VI are supplied to the respective systems. The coupling capacitors 1, 2 and 3 are provided to eliminate only the DC components of the color signals R, G and B since the respective DC components are not constant. The AC component of each of the color signals R, G and B, in which the DC components are eliminated by the capacitors 1 to 3, varies in black level according to the kind of a picture signal in a state of leaving as it is (for example, an inverse character display and a normal character display are different in black level), and therefore it is difficult to perform a later-stage process such as a gamma correction and the like. Thereupon, in order to prevent the black level of a picture signal from varying, pedestal level clamping circuits are provided for reproducing the DC components to the AC components of the color signals R, G and B. The foregoing circuits are called the clamp units 400, 500 and 600.
The processing system of the color signal R is explained as a representative in the following. The clamping voltage generating circuit 420 in the clamp unit 400 generating a divided voltage FR corresponding to a desired clamping voltage CR by dividing a power voltage VDD by means of the variable resistor 423. The buffer circuit 422 buffer-amplifies the divided voltage, generates and supplies a clamping voltage CR to a clamping portion 410. The clamping portion 410 operates so as to superpose the supplied clamping voltage CR on the pedestal level of the AC component of a color signal R, namely, the black level of a color signal R and outputs a clamped color signal RC. This is a clamping operation.
In the same way, clamped color signals GC and BC are outputted by performing clamping operations in the systems of color signals G and B.
In such a way, in a conventional clamping circuit the clamped color signals RC, and GC and BC can set their black levels independently. In this example the clamped color signals GC and BC are interlocked, but they can be composed so that they can be set independently of each other. However, once these clamped voltages, namely, the black levels are set, their settings are fixed as they are.
On the other hand, the black levels to be set of the gamma-correction circuits 7, 8 and 9 at a later stage of the clamp units 4, 5 and 6 are ideally set aiming at a predetermined voltage. As a matter of fact, however, the black levels vary among signals R, G and B or even among the gamma-correction circuits of the same color due to variation in accuracy of the gamma-correction circuits.
As shown in FIG. 3 showing an example of the input/output characteristic of a gamma-correction circuit, however, when the black level of an output of a clamping circuit is fixed, the black level of the output of the clamping circuit may become different from the black level of the gamma-correction circuit. In such a state, therefore, a gamma correction cannot be correctly performed.
If the black level of a gamma-correction circuit varies, a circuit at a later stage of the gamma-correction circuit results in operating incorrectly due to that variation and for example the black level results in being corrected slightly toward the white side. As the result, a disadvantage that the display becomes whitish or poor in contrast occurs.
Moreover, since the clamping voltages are set individually but fixedly, there is a problem of deterioration in color reproductivity or degradation in gamma-correction curves due to a fact that the correction may become an inverse correction depending upon variation in a process at a later stage of the clamping circuit.
Since a clamping circuit for a conventional liquid crystal display device as described above can set a clamping level for each of color signals R, G and B but cannot change the clamping level so as to absorb variation in black level of gamma-correction circuits and the like at a later stage of the clamping circuit, it has a disadvantage that if the black levels of a later-stage circuit vary, the later-stage circuit operates incorrectly due to that variation.
Furthermore, since the clamping voltages are set individually but fixedly, there is a disadvantage of deterioration in color reproductivity or degradation of gamma-correction curves due to a fact that the correction may become an inverse correction depending upon variation in a later stage of the clamping circuit.
An object of the present invention is to provide a clamping circuit for a liquid crystal display device which performs prevention of a coloring phenomenon as well as optimization of a gamma correction due to a circuit composition and prevention of deterioration in contrast by making it possible to adjust the black levels of clamping levels and the black levels of circuits at later stages of the clamping circuits so as to absorb variation in black levels of these circuits including the later-stage circuits.
According to the present invention, a clamping circuit for a liquid crystal display device has a plurality of clamp units for receiving a plurality of input color signals of an input picture signal after deleting a DC bias from each of the input color signals to produce a plurality of clamped color signals, respectively, by adding a predetermined clamping voltage corresponding to a pedestal level of the input picture signal to each of the input color signals. Furthermore, a plurality of gamma-correction circuits are connected to the clamp units for receiving the clamped color signals, respectively, and for performing a predetermined gamma correction and amplification on each of the clamped color signals to produce output color signals, respectively. Each of the clamp units is provided with a clamping portion for receiving the input color signal and a clamping voltage generating circuit for supplying a clamping voltage to the clamping portion so as to be controlled in response to an individual first control signal supplied thereto and generates the clamping voltage such that a black level of the clamped color signal coincides with a black level in an input/output characteristic of the gamma-correction circuit. The clamping portion is supplied with the clamping voltage so as to add the clamping voltage to the pedestal level of the input color signal at a predetermined timing in response to a second control signal supplied thereto.
The pedestal level may be a level corresponding to the black level of an input picture signal, or the black level of a picture signal may correspond to a setup level which is slightly higher by a certain voltage than the pedestal level.
The clamping voltage generating circuit may be provided with a D/A correction circuit for performing a digital-to-analog (D/A) correction on a digital data signal contained in the first control signal and outputting an analog voltage signal, and a buffer circuit for buffer-amplifying the analog voltage signal and outputting the clamping voltage.
The clamping portion may be provided with a switch circuit for conducting or shutting a clamping voltage in response to the level of the second control signal.
Furthermore, it may be provided with a control means for outputting a first and a second control signal, and a memory means for memorizing data from said control means and reading and supplying the memorized data to the control means.
The first control signal may be a serial control signal of a 3-line control method, and may have a digital data signal corresponding to the clamping voltages so as to be supplied to three control lines thereof, a clock signal for synchronization of this digital data signal and a strobe signal for specifying a clamping voltage generating circuit to be controlled.
The second control signal may be a signal which comes to be at high level xe2x80x9cHxe2x80x9d for only a specific period of the pedestal level of a color signal for one horizontal scanning period which is one period of a horizontal synchronizing signal, and may be generated by shifting the phase of the horizontal synchronizing signal by a predetermined time and inverting this shifted horizontal synchronizing signal.
According to the present invention, a clamping circuit of a liquid crystal display device is provided with a clamping circuit for adding newly a predetermined clamping voltage corresponding to the black level corresponding to a setup level which is a level higher by a certain voltage than the pedestal level of a picture signal to a color signal having a direct current (DC) bias cut for each color signal of the inputted picture signal and outputting a clamped color signal, and a gamma-correction circuit for performing a predetermined gamma correction and amplification on the supplied clamped color signal and outputting an output color signal, wherein a clamping circuit for each color signal is provided with a clamping voltage generating circuit which is controlled in response to the individual first control signal and generates a clamping voltage such that the black level of a clamped color signal coincides with the black level in an input/output characteristic of a gamma-correction circuit, and a clamping portion which is supplied with a clamping voltage and adds the clamping voltage to the pedestal level in a predetermined timing of a color signal in response to the second control signal.
And in case that the black level of a picture signal corresponds to a setup level higher by a certain voltage than a pedestal level, the clamping voltage generating circuit may set the clamping level at the black level by setting the value of a digital data signal corresponding to a clamping voltage contained in the first control signal lower by the setup level portion than the pedestal level.